(a) There is known a thread forming tap that has a working portion provided with an external thread corresponding to an internal thread to be formed. On the external thread, a plurality of margin portions forming the internal thread, and a plurality of relief portions having a smaller diameter than the margin portions, are disposed so as to respectively extend in an axial direction in parallel to a tap axis and alternate around the tap axis. (b) By screwing the working portion into a prepared hole, the margin portions cuts into a surface layer of the prepared hole to form the internal thread to thereby plastically deform the surface layer.
In such a thread forming tap, due to a lead angle of the external thread, a leading margin M1 disposed at a leading flank, is shifted in a screw-rotation direction in comparison to a following margin M2 as shown for example in FIG. 2B. The following margin M2 is disposed opposite to the leading margin M1, relative to a crest.
Because when the thread forming tap bites into i.e., chamfer a worked body, the leading margin M1 cuts into the surface layer of the prepared hole ahead of the following margin M2, an action by which the thread forming tap is pushed back acts and a tapping torque is thereby increased. This is due, in a case of forming the external thread on the thread forming tap by grinding by a grinding abrasive, to inclining the grinding abrasive by just the same lead angle as the external thread. When an inclination angle of the grinding abrasive is the same as the lead angle of the external thread, the leading margin M1 and the following margin M2 become symmetrical shapes with respect to a root, and the leading margin M1 leads the following margin M2 by just the lead angle.
Meanwhile, in an art proposed in Patent Document 1, in order to reduce the tapping torque, margin portions are disposed so as to be inclined at the same angle as a lead angle of a thread in an opposite direction, and a following margin is made to lead around an axis. In FIG. 2B, the shaded portions show the margin portions M.
[Patent Document 1] Japanese Published Unexamined Patent Application No. 2003-127027